This invention relates to drive arrangements for motors and generators which are subject to internal wiring faults.
Motors and generators, referred to herein as "electric machines", are typically arranged with phase windings which are connected internally in a wye or delta configuration. These connections provide a low impedance path for fault current to flow in the event of an inter-phase fault. Similarly, other winding arrangements that do not provide any isolation between phases result in a low impedance path for inter-phase fault currents to flow. For example, when a phase-to-phase short occurs because the insulation between two phase conductors deteriorates or when two or more phases are connected externally during or as a result of maintenance work high fault currents are produced through the low impedance paths between phases.
For a bolted phase-to-phase fault, fault current contributes to the fault from the source since a continuous circuit through the fault is provided by the interconnections of the phases at the source. Fault current is contributed to faulted electric machine stator windings because of the continued rotation of a permanent magnet field within the faulted stator winding. Such fault current flows through the fault because a continuous circuit is provided by the interconnected phases of the windings.
Electric machines with controllable fields such as wound field synchronous machines have the ability to limit the duration of fault current contribution by de-energizing the magnetic field. Other machines, such as the permanent magnet type cannot limit duration of fault current contributed by the machine during a coast down subsequent to power circuit tripping as a result of the fault. A three-phase fault is similar to the above described phase-to-phase fault except that it involves all three phases instead of just two phases. Continued operation with a faulted three phase machine, if possible at all, would require increasing the machine leakage reactance which would require a trade-off with respect to normal performance characteristics.
A phase-to-ground fault occurs when motor insulation fails between a conductor and a ground such as a grounded motor structure. Phase-to-ground fault current magnitudes have previously been mitigated by utilizing a high resistance ground impedance between a multiple phase system neutral and ground. However, with typical delta or wye connected systems, this resistance to ground consists of a single resistor connected to the neutral of a wye system or the derived neutral of a delta system and the value of that resistance must be such that the impedance is less than the impedance resulting from the total system charging capacitance which, in a delta or wye system, consists of all three phases in parallel. This results in relatively larger amounts of available ground fault current for a single phase line-to-ground fault than for a single phase circuit. Moreover, grounding arrangements in such systems have no ability to detect phase faults.
Interphase fault detection arrangements have been disclosed in the prior art. For example, the patent in Hasegawa U.S. Pat. No. 4,363,065 discloses an interphase fault detection arrangement in which each phase has independent detection windings and includes a corresponding ground resistor in which fault detection is dependent upon voltage sensing across each ground resistor. The Patent to Gale et al. U.S. Pat. No. 5,587,864 discloses protection circuitry for short circuits and ground faults utilizing phase current sensors and differential sensors. The Baker et al. U.S. Pat. No. 5,521,787 discloses differential current fault protection utilizing both single and multiphase current sensing and the patents to Traub U.S. Pat. No. 3,584,259 and Lardennois U.S. Pat. No. 3,999,104 are directed generally to phase fault sensing utilizing independent resistors connected to ground.